The present invention relates generally to apparatus for coupling a rotatable driven input shaft to an output shaft for imparting rotary motion to the output shaft. The present invention particularly relates to a two-speed transmission unit for rotating selectively the output shaft at two different speeds and which is also constructed to provide for free wheeling of the output shaft in one mode of operation and for fail-safe braking of the input and output shafts under certain conditions.
Two-speed transmission units for coupling a rotatable input shaft to a rotatable output shaft are well known in the art. Known types of such units include a planetary gearing arrangement which includes a sun gear, one or more planet gears carried on a planet carrier, and a ring gear. Clutches are provided for clutching various parts of the planetary gearing to the output shaft and to each other for purposes of effecting the two-speed drive to the output shaft. A variety of different arrangements are well known and a typical one of these arrangements is shown in U.S. Pat. No. 3,436,986.
According to U.S. Pat. No. 3,436,986 a pair of sun gears are rotatably supported by an input shaft. Separate transmission gear arrangements are associated with each of the sun gears, each transmission gear arrangement including a pinion gear coupled to each sun gear and adapted to couple its respective sun gear to a rotatably supported output shaft to rotate the output shaft at a selected angular velocity. This is a fairly complicated structure requiring numerous part and employing a substantially sized housing to enclose all of the parts.
According to the present invention, a two-speed transmission unit comprises a sun gear fixed to a rotatably supported input shaft and rotatable therewith about the central axis of the input shaft. One or more planet gears are disposed for orbital motion around the central axis of the sun gear, and the planet gears are connected to a planet carrier which fixes the planet gears for orbit about the central axis of the sun gear. First clutch means are provided for coupling the sun gear to the output shaft for rotating the output shaft at a first angular velocity equal to the angular velocity of the sun gear, and second clutch means are provided for coupling the planet carrier to the output shaft for rotating the output shaft at a second angular velocity equal to the velocity of the planet carrier about the sun gear.
The first clutch means preferably takes the form of a series of friction disc members rotationally fixed to the output shaft, and another series of friction disc members rotationally fixed to the sun gear. Both sets of friction disc members are mounted for axial movement relative to the output shaft. First piston means are provided for selectively compressing the friction disc members against each other so that their friction surfaces engage each other to engage the clutch means and to transmit the rotary motion of the sun gear to the output shaft.
The second clutch means also preferably takes the form of a series of friction discs rotationally fixed with respect to the output shaft, and a another series of friction discs rotationally fixed to the planet carrier. Second piston means are provided for selectively compressing the second set of friction disc members against each other to urge their friction surfaces against each other to engage the clutch means to transmit the motion of the planet carrier to the output shaft.
In another feature of the present invention means are provided for selectively placing the first and second clutch means in a condition in which (1) one clutch means is selectively engaged to transmit rotary motion to the output shaft while the other clutch means is selectively disengaged, or (2) both clutch means are disengaged in order to allow the input shaft to be rotated without transmitting any rotation to the output shaft (i.e., the input shaft "free wheels"). neither clutch means is disengaged, the system is designed to place both clutch means in engaged condition, thereby providing for fail-safe braking of the input and output shafts.
In the preferred embodiment, spring force applying means are provided between the pistons associated with the first and second clutch means and are adapted to continuously urge both of the pistons in directions tending to cause engagement of their respective clutch means with its respective gear (or planet carrier) and the output shaft. Fluid pressure applying means are provided in connection with each piston and are selectively operative to hydraulically urge either piston in a direction against the bias of the spring force applying means, to thereby disengage the clutch means. Whenever the fluid pressure applying means are not effective to disengage either clutch means, the spring means places both clutch means in engaged condition, thereby providing for fail-safe braking of the system.